A US coroner has revealed that troubled American actress Hayden Panettiere was in cardiac arrest when first responders arrived on scene.

The Heroes star’s family announced their “unimaginable” loss after Panettiere, aged 36, was found unresponsive by a friend at her apartment in South Carolina on Sunday (local time).

The Greenville Coroner’s Office and Greenville Police Department shared details of her death.

“First responders and Emergency Medical Services arrived on scene and located a female in cardiac arrest,” said a statement.

“EMS personnel initiated advanced cardiac life support measures; however, resuscitation efforts were unsuccessful, and the individual was pronounced deceased at 2.32pm.”

The coroner said a preliminary autopsy had found “no signs of trauma” that would have contributed to Panettiere’s death.

“The cause and manner of death remain pending further investigation and the completion of additional studies.

“This investigation remains active and ongoing.”

Meanwhile, audio of an emergency call suggested Panettiere was found dead due to a suspected overdose.

The TMZ website has obtained dispatch audio of the call to emergency services, which appears to pick up officials discussing an “overdose”.

The short recording includes one officer saying: “Responding. Overdose. CPR is in progress.”

After receiving the emergency call, paramedics rushed to the apartment and tried to resuscitate Hayden using “advanced cardiac life support”.

But after an hour, Nashville star Hayden was pronounced dead.

Actress Brittany Snow pays tribute to Hayden Panettiere. Photo: Instagram

Panettiere’s representative Kasey Kitchen also confirmed officials were looking into what happened and said more details would be revealed.

“There’s an investigation ongoing. We should know more tomorrow,” she told NBC News.

Panettiere’s death was confirmed by her father, Alan Panettiere, who hailed the actress as an “incredible light and a force of nature”.

“It is with profound sadness that we share the tragic passing of our beloved Hayden,” he said.

“She was an incredible light and a force of nature who brought immeasurable love and joy to all who knew her — and to the millions who watched her onscreen.”

Her father asked for privacy “as our family takes time to process this unimaginable loss”.

Panettiere recently released her memoir, This Is Me: A Reckoning, in which she detailed her troubled personal life.

Her traumas included domestic abuse, battles with alcoholism, stints in rehab and a battle with undiagnosed postpartum depression after the birth of her daughter Kaya.

Promoting her memoir back in May, Panettiere appeared on CBS with interviewer Gayle King.

“It was hard when she was here that day,” King said on Sunday.

“When she was here in the studio that day, and I was talking to her off-camera, she was saying she felt that she was in a good place,” the anchor shared.

“But she was still, clearly still struggling. To wake up to hear this news was really tough.

“She was so good. She would’ve turned 37 on Friday.”

Panettiere signed over custody of her daughter to her ex-fiancé, former champion boxer Wladimir Klitschko, in 2018, saying the legal battle would have destroyed her psychologically.

“I remember every second of that day. Every f—ing second,” she wrote in her memoir.

“No mother could forget a single thing about the day she signed her child away.”

-with AAP